For the original oil I am not sure, but at my company we make our own shock absorbers for weigh scales
Maybe another user knows exact what te original oil was, but I have spend some time by self building and modification from shock absorbers.
My '51 Morris Minor needs mineral oil grade 20, except for cold conditions.
At the company, we are using hydraulic oil, and because of the food, it is the Shell Cassida food-grade type.
But standard hydraulic oil can be used when no food is there
The working temperature is important, when it is a hot or cold country where the machine has to run, we are using between grade 15 and grade 45.
Default grade 30., this has to do with the small movement from a few mm, and the construction.
So the gap between piston and cylinder is important, together with the movement from the piston.
When it is only a few cm, thicker oil can be used, when it is a longer stroke, use thinner oil.
If there is a rubber O-ring inside, a standard ring from a hydraulic company can be used, with only metal parts inside, the type of oil is not very important.
Gear box oil, or standard engine oil can do the job also.
To check if the grade is correct, just fill it and check if the movement is normal.
The height of the level should be enough for a few cm higher as the top position for the piston.